As described in another report, the studies of protein folding in this Section have led to the hypothesis that (1) a previously unknown non-covalent interaction exists in the hydrophobic cores of proteins; (2) this new interaction, called the core loop interaction, is exothermic and mediated by the contacting groups which form a closed loop in the core; and (3) the core loop interaction is sensitive to the detail of the group contact and therefore has the ability to order the core groups. Antigen-antibody interaction resembles this core loop interaction in two critical aspects. First, both the core loop interaction and antigen-antibody interaction recognize a single amino acid substitution even if no polarity - change is involved. Second, both the core loop and the interface between antigen and antibody are devoid of solvent. Thus, we speculate that the specificity of antigen-antibody interaction has its origin in the core interaction loop which would form within or across the antigen antibody interface. To test this idea 6 hybridoma cell lines producing IgG monoclonals to yeast iso-1-cytochrome c have been prepared in the previous years. Our strategy is that (1) cloning the cDNA of the individual monoclonals; (2) sequencing of the CDNA to deduce the amino acid sequences; (3) constructing the three dimensional structures of monoclonals by computer homology modelling to examine the antigen binding sites; (4) developing an expression system for the cloned cDNA; (5) identifying the hydrophobic cores of the domains of the monoclonal molecules by computer modelling; (6) mutating the core residues (one at a time) by site directed mutagenesis; (7) expressing the mutated CDNA to examine the influence of the mutation on antigen-antibody interaction; and (8) if the results of (7) are positive, (7) will be repeated with different core residues of the same domain and also with the core residues of different domains to map the groups which influence antigen-antibody interaction. If the hypothesis is correct, the core groups remote from the antigen binding site should influence the antigen-antibody interaction. In the current year cloning cDNA prepared from mRNA of hybridoma cell line 2-96-12 (one of the above 6 hybridoma cell lines) has been carried out The cDNA library (phages) thus obtained was screened for the presence of clones containing the kappa light chain cDNA. Thus, 37 positive clones were purified.